The influence of starting microstructure on the retention and mechanical stability of austenite in an intercritically an
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I.
INTRODUCTION
T H E presence of retained austenite (yR), which is often inherent to the steel processing by intercritical (I/C) annealing, in a commercial ferrite plus martensitic (c~ + c~') dual phase steel could be beneficial to dual phase steel strength and ductility in a manner similar to the one found in metastable austenitic, the so-called TRIP, steels, l Results of a recent theoretical investigation by our group 2'3 indicate that increasing volume fraction of YR of optimal mechanical stability enhances the ductility of dual phase steels. However, no systematic study has so far been conducted to determine the retention characteristics of austenite during I/C annealing of such steels with the objective of optimizing the conditions under which increased amounts of austenite of optimum mechanical stability can be retained. As part of a broad research program to develop superior dual phase steels, a systematic experimental study has been undertaken to identify the factors that govern the retention of austenite during cooling and its mechanical stability upon straining. Effects of such variables as annealing time, temperature, and starting microstructure which are known to influence the formation of austenite during reaustenitization4'5'6 and as such may influence the retention of austenite during cooling, have been examined in this paper. Some preliminary results are presented and discussed.
II.
EXPERIMENTAL
The alloy studied in the present work was a commercial line-pipe steel containing (by wt pct) 0.07 C, 1.63 Mn, 0.018 S, 0.012 P, 0.11 Si, 0.062 V, 0.024 Nb, 0.34 Cu, and 0.038 Mo. The calculated Al and A3 temperatures for this steel, as calculated using Andrew's formulae, 7 are -708 ~ and - 8 6 7 ~ respectively. Three different kinds N. C. GOEL, Research Associate, and K. TANGRI, Professor and Head of Metallurgical Sciences Laboratory, are with the Department of Mechanical Engineering, University of Manitoba, Winnipeg, MB, Canada. J.P. CHAKRAVARTY, formerly Master's Student at the University of Manitoba, is a Ph.D. Student at the Metallurgy Department, The Ohio State University, Columbus, OH. Manuscript submitted December 11, 1985. METALLURGICALTRANSACTIONS A
of starting microstructures, namely (i) fen-ite plus pearlitic (F + P), (ii) fully martensitic (Q), and (iii) tempered martensitic (QT) were produced by giving appropriate heat treatments (Table I) to 6 mm thick hot rolled line pipe steel sheets. Heat-treated materials were surface ground from both sides to a thickness of 3 mm to remove any decarburized layer. Intercritical annealing of tensile specimens (gage length 12.7 mm, cross section area 4.5 x 3 mm 2) machined from steels with (F + P), Q, and QT structures was carried out between 732 and 820 ~ for times up to 5 minutes in a salt pot such that the time for heating to temperature was approximately 1 minute. Post anneal cooling was done in still air (cooling rate --- 5 ~ except for a few samples that were quenched in oil or water to see the effect of cooling rate on austenite retention. The m
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